We get this question often lately. Here's the scenario:
I was trying to re-size or copy my UEFI (Unified Extensible Firmware Interface) for one reason or another and now I can't boot my Windows 8 or 8.1 PC.
There are some free software solutions out there some seem to work some don't. Most often when we receive this question the user has tried to use the Windows Copy command and the Windows Disk Management Tool (WDMT) to copy the UEFI partitions contents to larger partition, then uses the WDMT to delete the old UEFI partition then re-size the new one, set the new UEFI partition to active and then re-boot. This is were the trouble starts. At this point the BCD store is corrupted and the system will not boot. Here is a fix that I hope will help.
Firstly, boot from a Windows 8 recovery drive (CD/DVD/USB). If you don't have a recovery drive (and this is likely because most new machines don't come with one), then you will either have to install the drive into another Windows 8 machine or obtain a Windows 8 recovery drive, or boot from the recovery partition of the boot drive (If this option works at this point). Don't even bother with the automated recovery process, it will not work as it cannot find a Windows partition.

You will need to choose the language and time settings.

Choose 'Repair Your Computer'.

Choose 'Troubleshooting'.

Choose "Advanced Options'.

Next choose 'Command Prompt'.

Next, we're going to use the DISKPART tool to verify that the UEFI partition has a drive letter assignment.
Enter 'DISKPART' (Enter)
(In this test case we removed all other drives except the boot HDD and DVD, so we know the disk 0 is our boot HDD)
DISKPART> sel disk 0
Disk 0 is now the selected disk.
DISKPART> list vol
There was no drive letter assignment to our UEFI partition (volume 3) so we need to assign a drive letter.
DISKPART> sel vol 3
Volume 3 is the selected volume.
DISKPART> assign letter=E:
DiskPart successfully assigned the drive letter or mount point.
Exit DiskPart tool (Enter EXIT)
Next, we need to change to the boot folder on the UEFI volume.
Change to the UEFI volume into the boot folder. Depending on the way Windows was installed this path maybe one of the following.
cd /d E:\Boot\
cd /d E:\EFI\Microsoft\Boot\
cd /d E:\ESD\Windows\EFI\Microsoft\Boot\
Now we will need to enter three command lines to repair the BCD store. Again depending on the way Windows was installed these may not all be necessary, however
command lines are are unnecessary will not affect the outcome.

bootrec /fixboot

bootrec /fixmbm

That's it, if all went well the BCD store should be repaired, and the system will once again boot.